To develop international law claims, it is often critical to compare different countries’ laws. This essay explores how methods drawn from comparative law and comparative politics research can help international lawyers make comparative inquiries more simply and straightforwardly.
International lawyers recognize three main sources of legal authority: treaties, custom, and general principles. Cross-national comparisons are deeply embedded in the very definitions of two of these three sources. To establish international custom, an international lawyer must show that a broad range of states consistently engage in a certain practice out of a sense of legal obligation. To establish a general principle, an international lawyer must show that it is “recognized by civilized nations”; in practice this requires that the principle be found in diverse legal families. Treaty interpretation does not necessitate cross-country comparison as a matter of definition: in theory, the text of the treaty itself could provide the requisite answers. However, in practice, international and domestic courts are typically faced with ambiguous treaty terms. To interpret them, they often turn to the jurisprudence of diverse states and to subsequent state practice, thus implicitly beginning a comparative inquiry. in sum, comparative international law is useful for identifying and applying international law, as this volume’s introduction explains.